1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.
2. Description of the Related Art
An example of such an air-fuel ratio control apparatus for an internal combustion engine is disclosed in Japanese Unexamined Patent Application Publication No. 9-317531. An exhaust path of the internal combustion engine is provided with a catalyst for cleaning up exhaust gas. A linear air-fuel (LAF) sensor and an oxygen concentration sensor are provided upstream and downstream, respectively, of the catalyst. The LAF sensor linearly detects an air-fuel ratio of exhaust gas (hereinafter may be referred to as an exhaust gas air-fuel ratio). The oxygen concentration sensor detects an exhaust gas air-fuel ratio, and is of so-called inversion type having an output characteristic that changes dramatically around an exhaust gas air-fuel ratio corresponding to a theoretical air-fuel ratio.
This air-fuel ratio control apparatus feedback-controls a fuel injection quantity such that the exhaust gas air-fuel ratio detected by the LAF sensor is equal to a target air-fuel ratio. Also, the air-fuel ratio control apparatus calculates the amount of correction for correcting the fuel injection quantity in accordance with the exhaust gas air-fuel ratio detected by the oxygen concentration sensor. Specifically, a range of output values of the oxygen concentration sensor is divided into four regions by a target value corresponding to the target air-fuel ratio, and two values, one corresponding to a predetermined rich reference air-fuel ratio richer than the target air-fuel ratio and the other to a predetermined lean reference air-fuel ratio leaner than the target air-fuel ratio. Then, if the output value of the oxygen concentration sensor is in one of the two regions closer to the target value, the amount of correction is set to a smaller value, and if the output value of the oxygen concentration sensor is in one of the two regions farther from the target value, the amount of correction is set to a larger value. Thus, a larger amount of correction is used for regions where an output deviation representing a difference between the target value and the output value of the oxygen concentration sensor is large, and thereby the exhaust gas air-fuel ratio is quickly adjusted to the target air-fuel ratio.